This invention relates to methods and apparatus for remotely monitoring the performance of medical electronic devices and more particularly an apparatus and method for remotely monitoring the performance of an implanted cardioverter defibrillator.
An implanted defibrillator is a pacemaker-like device that senses intrinsic cardiac rhythm. If the device determines that a rapid, life threatening ventricular tachycardia or ventricular fibrillation is present, a battery within the device charges a large capacitor, and then discharges the capacitor to deliver a defibrillatory shock to the patient for the purpose of returning the heart to normal rhythm.
One such device is known as an automatic implanted cardioverter defibrillator (AICD) and is manufactured by Cardiac Pacemakers Inc., St. Paul, Minn. This device determines the presence of ventricular tachycardia or ventricular fibrillation by using programmable rate criteria. That is, a detected rate greater than 150, 160, 170, etc. will be classified as ventricular tachycardia or ventricular fibrillation. This device is connected to the heart via four electrodes, two of which are similar to pacemaker electrodes. The device attempts to sense the patients ecg via these electrodes, thereby determining the heart rate. The other two electrodes are used to deliver the defibrillatory shock.
The proper sensing by the device is monitored by obtaining a "beep-o-gram". When a magnet is placed over the device, it will admit a 3200 Hz, 50-100 msec duration audible beep in response to every ecg QRS complex sensed. The monitoring physician performs a beep-o-gram by connecting the patient to a surface ecg; and, while watching the ecg, listening for a beep coincident with the observation of each QRS complex. The beep can be simultaneously recorded on a two channel chart recorder with the surface ecg to visually display and make permanent records of the beep-o-gram. Additional information concerning beep-o-grams may be found in the paper entitled "The Use of Beep-O-Grams In the Assessment of Automatic Implantable Cardioverter Defibrillator Sensing Function", Ballas, S. L., et al., PACE, Volume 12, pages 1737-1745, November, 1989, which paper is incorporated by reference as if fully set forth herein.
Oversensing can be determined if more than one beep is present per QRS complex, or if a beep occurs with each QRS and T wave. Oversensing of this type, called double sensing, would fool the device into thinking that the heart rate was twice what it really is, causing inadvertent shocks if the patients heart rate was more than 75-90 beeps per minute.
Undersensing can be determined if no, or intermittent beeps are heart coincident with QRS complex. Such might be due to a loose electrode, poor electrode contact, high electrode impedance or device malfunction. In any case, undersensing can cause the device to miss the episode of ventricular tachycardia or ventricular fibrillation, thereby denying the patient defibrillatory therapy when it is needed.
The current generation of implanted cardioverter defibrillators can be turned of by applying a magnet over the device for greater than 30 seconds but less than 60 seconds. Thus, in order to obtain a beep-o-gram, the magnet must be applied to the device for less than 30 seconds to avoid shutting the device off.
Pacemaker patients are routinely followed conveniently at home by issuing them telephone ecg transmitters. The device connects to the patient by two electrodes, amplifies the ecg, converts the ecg via voltage to frequency modulation centered at 1500-2000 Hz for telephone transmission. A frequency to voltage demodulator in the pacemaker follow up company location is used to demodulate the ecg for display to a monitor technician or physician.
Pacemakers remain inhibited if the patients intrinsic heart rate is greater than the pacer's programmed rate. Pacemakers pace the heart, displaying a pacing artifact in front of every paced QRS complex on the surface ecg, if the patients heart rate falls below the pacemakers programmed escape pacing rate. Thus, proper sensing can be determined by seeing that the device does not pace when the patients heart rate is adequate, and does pace when not adequate. To expedite the telephone monitoring, a magnet placed over the pacer will cause it to pace even if heart rate is adequate, thereby enabling determination of pacemaker capture. Capture is defined as whether the output of the pacer is sufficient to pace the heart, which is determined by seeing a QRS complex initiated and following every pacing artifact.